The share of machine dishwashing tablets in certain markets has grown significantly in recent years primarily because they are perceived to be more convenient than alternative product forms such as powders. However, the product form and method of delivery of tablets can limit both the type of functional ingredients incorporated and the level of functionality from these ingredients.
A complication unique to tablets derives from the method of introduction into the machine. Thus, some tablets are designed to be placed directly into the machine itself, such as in a basket hanging from the upper rack, where they come into contact with a water spray as soon as the machine starts, while others are delivered via the dispenser and are only released during the main wash cycle. Clearly, the release and performance of functional ingredients will differ depending on how the tablet is delivered.
Each type of delivery has potential weaknesses. Thus, for tablets that come into immediate contact with the water spray, some of the functional ingredients can be released into the pre-wash where, if the temperature is too low the ingredients will be lost without delivering a significant benefit. For both types of tablets, complete dissolution may not occur during the main wash cycle. If part of the tablet is still available for dissolution in the rinse, serious spotting and filming problems can occur. These potential negatives are specific to the tablet form. Liquids or powders are introduced into the wash via the dispensing cup and so there are no losses during the pre-wash and the rapid rate of dissolution of these products ensures no carry over of undissolved product into the rinse. Current tablet technology is not consistently successful in meeting the performance standards of other product forms by ensuring that all the functional ingredients are delivered during the appropriate part of wash cycle.
Prior art attempts to optimize the performance of tablet technology have primarily been directed towards modification of the dissolution profile of tablets. This is deemed especially important for those tablets that are placed in the machine such that they come into contact with a water spray at the very beginning of the wash process. A number of patents suggest technology to minimize dissolution in the pre-wash to allow the maximum amount of functional ingredients to operate in the main wash. In particular, a 2-layer tablet for machine dishwashing is described in EP 224128. Both layers contain metasilicate and triphosphate but by modifying the degree of hydration one layer is cold water soluble while the other layer dissolves rapidly at increasing temperatures.
Similarly, EP 224135 describes a combination of a cold water-soluble melt or tablet with a cold water-resistant melt or tablet that is soluble at increasing water temperatures. The cold water-soluble melt composition consists of a mixture of metasilicate monohydrate, pentahydrate and anhydrous metasilicate and the cold water-resistant tablet layer consists of metasilicate nonahydrate and triphosphate. EP 224136 describes similar compositions in the form of multi-layer fused blocks in which the layers have different dissolution rates. One layer consists of metasilicates having different degrees of hydration and another layer consists predominantly of sodium metasilicates and anhydrous sodium tripolyphosphate.
Phosphate-free tablets containing a combination of metasilicates, a low foaming surfactant, sodium acrylate, sodium carbonate, sodium sulfate, a bleaching agent and water are described in WO 9115568. These tablets are claimed to be 10-40% soluble in the cold water pre-rinse leaving 60-90% for the main wash.
WO 9300419 describes production of phosphate- and metasilicate-free tablets. Anhydrous sodium carbonate and optionally other builders are mixed with acrylate and water sufficient for partial hydration of the anhydrous carbonate. The remaining components are added and the whole compressed into a tablet. The advantage is that the tablets only partially dissolve during the pre-wash stage so that greater than 50% is available for the main wash. Similar technology is described in DE 4112075.
A broad solubility profile for tablets is described in EP 26470. The tablets contain preferred ratios of anhydrous and hydrated metasilicates and anhydrous triphosphate, active chlorine compounds and a tabletting aid consisting of a mixture of sodium acetate and spray-dried sodium zeolite. Good solubility in warm water makes at least 65% of the tablet available for the cleaning stage of the wash.
DE 4229650 describes a tablet with rapid dissolution. Anhydrous sodium tripolyphosphate is partially hydrated to tripolyphosphate hexahydrate and the partial hydrate is mixed with powdered water-free silicate, sprayed with water or aqueous silicate, granulated and mixed with optional cleaning components. Tabletting auxiliaries sodium metasilicate pentahydrate and/or nanohydrate comprising of about 8-12% of the total granulate mix are included.
Thus, in terms of optimizing the performance of machine dishwashing tablets, the prior art primarily deals with traditional high pH formulations systems and suggested routes to improving the performance of tablets rely on modifying solubility profiles in a fairly coarse manner.
Regarding inclusion of acidity to enhance anti-scaling benefits, it is known that an acid source can be used in machine dishwashing compositions to remove scale which results from the use of hard water. For example, vinegar is utilized to remove hardness scale and many current dishwashing machine cleaning products use an acid source to remove scale buildup. Many rinse aids for dishwashing also contain citric acid.
The use of a detergent composition with a pH of less than 9.5 for enhanced filming performance is disclosed in WO 95/12653, However, these systems do not deliver the benefits of higher pH washing along with the benefits of low pH for good filming. At the pH where filming is perceivably diminished in the wash, poor performance is obtained from both protease and many bleach systems. WO 95/12654 deals with a similar system with a limitation on the ratio of calcium complexing component to carbonate source of at least 0.8. However, the problem of achieving good anti-scaling benefits, which are optimum at low pH, and good cleaning performance, which is optimum at a significantly higher pH, is unresolved.
WO 95/12657 describes this issue and discusses the application of delaying release of an acid source for improved spotting and filming for use in machine dishwashing powder compositions. However, methods of delaying release of acidity, claiming both use of poorly soluble coatings and of modifying the physical characteristics of the acid to control its solubility and rate of release are not specifically addressed. In addition, there is no reference to use in tablets.
Thus, one object of the present invention is to utilize the unique characteristics of the tablet form to deliver both good bleaching from a oxygen bleach source, with or without a bleach catalyst, and good enzymatic protein soil removal along with good anti-scaling results by virtue of controlled release of a source of acidity. The specific release parameters for the source of acidity are primarily defined by the wash temperature.
Another object of the present invention is to provide tablets which are aesthetically pleasing and which are more consumer friendly by virtue of the virtual absence of fines on the tablet surface than tablets conventionally known in the art.